I had four of these running in my front room for a few weeks before selling them. I never really noticed them (other than turning the TV up a few notches) until I turned them off, which makes you think the room's gone quiet. Can you sleep with it on? Yes, depending on what kind of sleeper you are and how much background noise bothers you :). Some people have fish tanks that make a similar "hum" that bothers some and not others - it isn't loud or noticeable to be annoying, but you sure know when its off.
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